Solvents employed in formulations of paints and other coating materials in the past have been selected largely from aromatic and aliphatic hydrocarbons. These generally have not been recovered because of the necessity of using large quantities of air in the drying process in order to maintain a mixture of solvent and air which was outside the flammable range. Such large quantities of air made it impossible, or at least uneconomical, to recover the solvents which were present in such dilute concentrations in the effluent.
The increasing use of less flammable solvents, e.g. chlorinated solvents, such as 1,1,1-trichloroethane, which are more expensive than the hydrocarbon solvents, has made their recovery possible as well as an economic necessity. Further, some of these chlorinated solvents are subject to state and/or federal regulation as effluents to the air.
Known methods are carbon absorption, refrigeration, and liquid extraction. To remove solvent from carbon requires either inert gas and subsequent refrigeration (which is very inefficient since it is a once-through recovery system) or steam stripping in which expensive drying of the recovered solvent is necessary. Liquid extraction processes involve energy intensive separation by distillation.
The present invention makes the recovery of such solvents feasible. The invention is a novel method of solvent recovery that avoids flammability and contamination problems associated with known methods. The process is a closed cycle, air circulating, refrigerated recovery system. The system can be used for aromatic and aliphatic hydrocarbons as well as chlorinated hydrocarbons. The latter are preferred since moderate temperatures above 32.degree. F. can be used to recover the solvent, thus avoiding freezing any trace amounts of water which may be present.
This invention also offers a safe recovery method by allowing separation to occur at solvent concentrations lower than the lower explosive limit. While known methods also allow recovery at lower solvent concentrations, more energy is required than with the present invention.